In order to increase maintainability in the future, we should try to
make structures as self-contained and opaque to its users as possible.
Thus it is probably not a good idea to just typedef `git_graftmap` to
`git_oidmap`, as that will make it a lot harder in the future to extend
the API in the future, if this need ever arises.

Refactor the code to instead declare a real structure `git_grafts`,
which is completely opaque to its callers.